1. Field of the Invention
This invention relates to an exposure apparatus for manufacturing semiconductors which is provided with a projection optical system, and in particular to a projection exposure apparatus provided with a detecting device for detecting the reflectivity of a semiconductor wafer placed on a stage.
2. Related Background Art
In a projection exposure apparatus for producing semiconductor devices such as super-LSI by the photolithograph including the step of projecting the image of a pattern on a photomask or a reticle onto a semiconductor wafer having photoresist applied thereto at a predetermined magnification by the use of a projection lens, matching accuracy is mentioned as one of the important performances thereof. One of important factors which affect the matching accuracy is the magnification error of the projection optical system. The size of the pattern used in super-LSI is in the tendency toward minuteness and correspondingly, the requirement for the improved matching accuracy on the wafer is strengthening. Accordingly, the necessity of keeping the projection magnification at a predetermined value has become very high. Now, the magnification of the projection optical system is fluctuated in the vicinity of a predetermined magnification by a slight variation in the temperature of the apparatus, a slight fluctuation of the barometric pressure of the atmosphere in the clean room and the application history of energy rays to the projection optical system. For this reason, projection exposure apparatuses provided with a magnification correcting mechanism for finely adjusting the magnification of the projection optical system and realizing the predetermined magnification have recently made their advent. Specifically, magnification correcting mechanisms which vary the spacing between an object (reticle) and the projection lens, or change the lens spacing in the projection lens or adjust the pressure in a particular air chamber in the projection lens are known. Also, the focus (the position of the imaging plane) is moved due to the same fluctuation factors as the fluctuation factors regarding the magnification and therefore, projection exposure apparatuses having a focus correcting mechanism have also recently made their advent.
Now, among the aforementioned fluctuation factors of the imaging characteristic, as regards the influence of the application of energy rays to the projection optical system, the image of the pattern can be controlled to a predetermined state of projection by a method of adjusting the pressure in the projection lens in confirmity with the application history, as disclosed, for example, in Japanese Laid-Open Patent Application No. 78454/1985 which corresponds to U.S. application Ser. No. 656,777 filed on Oct. 1, 1984 and already allowed. However, this method takes into account only the influence of the exposure light from the light source entering the projection lens and does not at all takes into account the influence imparted to the projection lens by the reflected light reflected on the wafer and again entering the projection lens. That is, the quantity of energy rays transmitted through the projection lens and contributing to the imaging can be easily measured on the basis of the exposure conditions determined by the application time, the application intensity and the transmittivity of the reticle, but the quantity of energy rays returned to the projections lens by the reflection on the imaging plane cannot be measured, and this has led to the disadvantage that any variation in the reflectivity of the object (the wafer or the like) positioned on the imaging plane would cause the controlled imaging characteristic to deviate from a predetermined state.
This will now be described by reference to FIG. 5 of the accompanying drawings. FIG. 5 shows the variations with time in the magnification on the wafer and the control pressure value during the pressure adjustment of the air chamber in the projection lens. It is to be understood that during time t.sub.1 to time t.sub.2, wafers of low reflectivity are successively exposed and from time t.sub.3, wafers of high reflectivity are exposed. Assuming that with regard to the wafers of low reflectivity, the quantity of energy rays (reflected light) returning to the projection lens is small and the influence thereof upon pressure control is negligible, the pressure value varies from P.sub.0 to P.sub.1 during time t.sub.1 to time t.sub.2 and is saturated at P.sub.1. During time t.sub.2 to time t.sub.3, no exposure is effected and therefore the pressure value gradually lowers from P.sub.1. Thereby, until time t.sub.3, the magnification of the projected image on the wafer is accurately controlled to a predetermined value M. Assuming that from time t.sub.3, exposure is effected under the same conditions as the previous wafers, the pressure value varies so as to be again saturated at P.sub.1. In this case, however, the quantity of energy rays is increased and therefore, under the influence of the reflected light thereof, the magnification deviates from the predetermined value M. Such a disadvantage has been found to reside in the aforedescribed projection exposure apparatus according to the prior art.
On the other hand, in a projection exposure apparatus, with the tendency of the patterns of semiconductor devices toward minuteness and higher integration, higher alignment accuracy as well as proper exposure control complementary to the sensitivity and thickness of the photoresist on the wafer is necessary. Heretofore, this exposure control has been designed by providing a shutter and the light-receiving portion of an integrating photometering device in the illuminating system so that the shutter is closed when a proper quantity of light has entered the light-receiving portion from an exposure light source. However, in such prior-art exposure control apparatus, which is of the type which detects the exposure light from the light source, exposure correction is possible for the fluctuation of the intensity of the exposure light, but the exposure correction for the difference in the photoresist on the wafer, particularly in the thickness thereof, cannot be accomplished, and this has led to the undesirable possibility that defect is created in the printed pattern.